3GPP, the 3rd Generation Partnership Project, develops specifications for third generation mobile phone systems, and also from Release 8 (Rel-8) the next generation specifications often referred to as LTE, Long Term Evolution.
Carrier aggregation is a proposed technology for future mobile phone systems in which multiple carriers are aggregated to increase the overall performance. For example LTE and HSPA (High Speed Packet Access) can be used in a single system, enabling the peak data rates of the two systems to be added together. The transition from UTRA (UMTS Terrestrial Radio Access) to E-UTRA (evolved UMTS Terrestrial Radio Access) will be gradual comprising several evolutionary steps. One scenario comprises use of HSDPA (High Speed Downlink Packet Access) and LTE, usually referred as LTE-HSPA CA.
In one proposed arrangement, all uplink transmissions would take place in the LTE system. The HARQ (Hybrid automatic repeat request) feedback and the ACK/NACK signals for the HSDPA transmissions in the downlink are transmitted in the uplink channel resources of the LTE.
The radio frame length is 1 ms for the LTE link and configurable between 10 ms or 2 ms for the HSPA link. As a result, the overall frame lengths are different in uplink and downlink. Particularly the downlink data transmission and the uplink HARQ feedback are asymmetric. This leads to a waste of channel resources, which is illustrated in FIG. 1.
The time scale is represented at the bottom of the figure, each tick being equivalent to 1 ms. The downlink data transmission is represented in capital letters. A single data transmission unit in the HSPA link takes 2 ms and in the LTE link takes 1 ms. For each data unit there exists a corresponding uplink resource in the LTE link for HARQ feedback, wherein HARQ feedback provides the ACK/NACK information. Each acknowledgement is represented in lowercase letters. A hash sign (#) is here used to describe a NACK, requesting a retransmission. As there are four component carriers for data transmission and thus four HARQ processes, there are also four HARQ feedback symbols in each radio frame. For example, HSPA component carrier A requires retransmission, arrow 2; whereas component carrier B is acknowledged, arrow 6. As a result, the component carrier A is retransmitted in the following downlink transmission, arrow 4.
The data in HSPA is transmitted only every 2 ms, which leads to every second acknowledgement resource being empty and thus an unused resource. One solution would be repeating the feedback and increase the reliability of the HARQ feedback signaling. However, this solution would not lead to a saving of resources.